There are some wafers in a box. The wafers can be put into plates of 2 or 4 with no wafers leftover. When the wafers are put into plates of 9, there are short of 3 wafers on the last plate. If there are more than 20 wafers but less than 40 wafers, how many wafers are there in the box?
If wafers can be put into plates of 2 or 4 with no wafers leftover, the number of wafers is a multiple of 4 as 4 is a multiple of 2.

Multiples of 4 that are greater than 20 but less than 40: 20, 24, 28, 32, 36, 40

Multiples of 9 that are greater than 20 but less than 40: 27, 36
Multiples of 9 (- 3) that are greater than 20 but less than 40: 24, 33

Common number = 24

Number of wafers = 24

Answer(s): 24